This invention pertains to a track search servo system in an optical disk drive, and more particularly to an improved method and apparatus for detecting track crossings during CD-ROM search operations.
Optical disk drive systems operate by storing and retrieving information on an optical storage medium at various track locations within the media. The system typically employs a track search operation in which a track is quickly located to store or retrieve the desired information. The search operation typically employs a process by which the number of track crossings are counted until the targeted track is reached. Using this technique, the distance traveled and the velocity of the head can be precisely controlled.
FIG. 1 shows a simplified block diagram of a conventional optical disk drive system 10. Optical disk 12, such as a CD-ROM is driven by spindle motor 14 under control of spindle motor control circuity 16. Optical pickup unit 20 retrieves or records information to/from disk 12 by means of photodiodes (not shown) properly placed above/below the desired track of the CD-ROM 12, the location of the pickup unit 20 being precisely positioned by feed motor 22. In addition, the pickup unit 20 provides a tracking error signal (TE) in order for the disk drive to maintain proper radial tracking.
Microprocessor 40 communicates with host computer 50 and, in turn, controls spindle motor 16, focusing servo 26, which in turn, controls the pickup unit 20 and tracking servo 24, which in turn, controls feed motor 22. The output signals from the pickup unit 20 are fed to focusing servo 26 and tracking servo 24 to aid in their operation, as well as the data processing circuity 30 for extracting data which is feed via BUS 44 to host computer 50, and signals for use by microprocessor 40 and spindle motor control circuity 16 for their operations.
The optical drive system typically operates in a coarse tracking mode initially to locate the desired track. During coarse tracking, microprocessor 40 calculates the difference between the current track of the CD-ROM 12 and the target track and determines the direction of movement. The difference calculated is the remaining distance the pickup unit 20 must travel to arrive at the target track, the difference being loaded into a counter in the servo system. A tracking servo 24 then drives the pickup unit 20 in the desired direction. The pickup unit 20 provides the tracking error signal (TE), which, when the pickup unit 20 is traversing the disk, is a sinusoidal waveform having a zero crossing whenever the pickup unit passes a track center. One cycle of the TE signal represents crossing of one track. Using the TE signal, the tracking servo system determines when the pickup unit crosses a track and decrements a counter by one.
The tracking servo continues to drive the pickup unit until the counter decrements to zero. At this point, the pickup unit reads the current track information and commences a fine search operation to arrive at the target track. It can be appreciated that the servo system must detect track crossing accurately so that the counter will decrement correctly. If, due to erroneous track crossing detection, the track counter miscounts, the pickup unit is grossly mis-positioned and the system has to re-seek, increasing the seek time significantly.
To generate accurate track crossing counts, prior art servo systems utilize the tracking error crossing signal TX and a quadrature signal RX. The TX signal is the digitized waveform of the tracking error signal TE. The quadrature signal RX is the digitized waveform of the radio frequency ripple signal RFRP. The RFRP signal is derived from the summed output of the photodiodes (within the pickup unit 20) which detects the reflected main laser beam.
When the pickup unit is positioned above/below a track center, the summed signal is the data signal and contains high frequency components. When the pickup unit is traversing the disk during the search operation, the summed signal becomes modulated. The modulation of this summed signal is a sinusoidal waveform 90.degree. out of phase with the tracking error signal TE. The RFRP signal is generated by filtering the high frequency components from of the summed signal, and when digitized, yields the RX signal.
Unfortunately, both the TX and RX signals are prone to noise contamination. This is especially true for the RX signal since it is derived from a data signal containing high frequency components. Noise and glitches in the RX and TX signals can lead to erroneous track crossing detection.
One commonly encountered problem is detection of multiple track crossings. This occurs when transients in the TE signal are recognized as additional zero-crossings which are translated into additional transitions when the TE signal is digitized into TX. Thus, multiple zero crossings can be erroneously generated for each legitimate track crossing. The erroneous TX signal generated causes the counter to miscount and the pickup unit to arrive at the wrong track.
What is needed is a servo system and method for accurately detecting the number of disk track crossings.